This invention relates to a high temperature electrical heating method and apparatus.
U.S. Pat. Nos. 4,640,352 and 4,886,118 disclose conductive heating of subterranean formations of low permeability that contain oil to recover oil therefrom. Low permeability formations include diatomites, lipid coals, and oil shales. Formations of low permeability are not amiable to secondary oil recovery methods such as steam, carbon dioxide, or fire flooding. Flooding materials tend to penetrate formations that have low permeabilities preferentially through fractures. The injected materials bypass most of the formation hydrocarbons. In contrast, conductive heating does not require fluid transport into the formation. Oil within the formation is therefore not bypassed as in a flooding process. Heat injection wells are utilized to provide the heat for such processes.
Heat injection wells can also be useful in decontamination of soils. U.S. Pat. No. 5,244,310, for example, disclose a method for decontamination of soils wherein heat is injected below the surface of the soil in order to vaporize contaminates. The heater of patent ""310 utilizes electrical resistance of spikes, with electricity passing through the spikes to the earth. Heating using the electrical resistance of elements which are in contact with contaminated soil results in a temperature profile which greatly depends upon the points at which current leaks to to the soil. Providing a uniform and predictable heat injection profile is therefore difficult.
U.S. Pat. No. 5,060,287 teaches an electrical resistance heater element for use in as a wellbore heater wherein the heating element is of a copper-nickel alloy, and is provided in a mineral insulated sheath. The copper-nickel alloy is particularly useful because the resistance as a function of temperature for this alloy is relatively flat. Thus, electrical resistance, and therefore heat release, at hot spots does not significantly increase. This cable heater design had disadvantages in that the surface area from which heat is transferred is small, and it is difficult to join segments of the heater cables to form spliced heater cables of significant length. It would be preferably to have a heater design which has a greater surface area from which to transfer heat, and which is easier to splice together long segments.
U.S. Pat. No. 2,732,195 discloses an electrical heater well wherein an xe2x80x9celectrically resistant pulverulentxe2x80x9d substance, preferably quartz sand or crushed quartz gravel, is placed both inside and outside of a casing of a wellbore heater. Heat must be transferred from the heater element to the casing conductively. It would be advantageous to provide such a heater wherein this heat could be transferred by radiation, which could significantly reduce the temperature difference required to transfer heat to the casing.
These and other objects are accomplished by a well heater effective for heating earth surrounding a wellbore from the wellbore, the well heater comprising: a) a resistive heating element, the resistive heating element traversing a segment of the wellbore to be heated; b) a plurality of ceramic insulators, the ceramic insulators defining a channel through which the resistive heating element passes; and c) a support element connected to at least one ceramic insulator, the support element effective for conducting heat from the ceramic insulator and radiating heat to the wellbore wall, and to support the weight of the electrical resistance element and the ceramic insulators through the connection to the at least one ceramic insulator.
The heater of the present invention may be utilized in applications such as soil remediation, recovery of oil from oil shales, tar sands, and diatomite formations. The heaters are simple, relatively inexpensive, and reliable. The support member preferably extends across at least 75% and more preferably at least 90% of the diameter of the well, so that heat conductively transferred from the heating elements through the ceramic insulators can be radiated to the inside surface of the wellbore from a greater surface area.